The present invention pertains to a four-wheel drive vehicle in which the wheels of one axle are driven by means of a clutch, especially a viscous clutch, which is located in the power train between the front and rear axles and which operates automatically as a function of the driving conditions.
A viscous clutch only transmits very small torques in the case of small speed differences between coupled shafts whereas the viscous clutch transmits very large torques at large speed differences. Depending on the location within the power train, the viscous clutch operates either as a central differential automatically locking as a function of the driving conditions or as a combined central differential and axle differential. During normal driving operation of the vehicle, there are only small differences in speed between the wheels of the two axles. The viscous clutch correspondingly transmits a small drive torque, and the drive characteristics essentially correspond to those of a two-wheel drive vehicle with only one driven axle. However, if there are very large differences in speed between the wheels, which occur, i.e., on wet ice-or snow-covered or gravel roads, the viscous clutch "locks-up", and depending on the location of the viscous clutch, the driving characteristics are those of a four-wheel drive vehicle with a locked central differential or with a locked central differential and a locked axle differential. Consequently, a permanently ready four-wheel drive is achieved with a viscous clutch.
However, the speed difference-dependent coupling achieved with the viscous clutch between the front and rear axles of a vehicle is not always desirable. During the operation and the maintenance of a four-wheel drive vehicle with a viscous clutch, there are a number of situations in which one would rather do without the torque coupling of the front and rear axles via the viscous clutch.
It was proposed before in DE-OS No. 33 17 247 that a controllable auxiliary clutch be mounted in the power train of a vehicle together with the viscous clutch to permit torque coupling and uncoupling of the drive. The prior art auxiliary clutch is designed as an overrunning grip roller and expanding friction clutch which can be automatically engaged and disengaged in terms of torque, or an hydraulically or electromagnetically operated disengaging clutch. It ensures that no torque is transmitted via the power train between the front and rear axles of the vehicle during operation of the service brake, so that the rear wheels are not locked during the braking of the front wheels.
The auxiliary clutch designs known from DE-OS No. 33 17 247 are complicated and bulky, and they cannot be easily integrated within the power train of a vehicle, and their control does not guarantee that the torque coupling of the vehicle axles is interrupted in all situations in which it appears desirable.